IMPORTANCE OF HUMUS 37 



sources of decaying vegetable matter are the plants them- 

 selves, either the roots and stubble which are left when the 

 crop is harvested or the entire plant which is turned under as 

 green manure, and barnyard manure. Barnyard or stable 

 manure is made up of bedding and parts of plants which 

 are not eaten by animals and also of the material in the food 

 they consume which they are unable to digest and assimi- 

 late, so that it is all vegetable matter. When this matter is 

 incorporated in the soil, it is acted upon by bacteria and 

 molds and reduced to forms in which it can again be used by 

 plants. Another important source of plant food and one 

 which is largely used in the Eastern and Southern states is 

 commercial fertilizers. These are composed mostly of 

 refuse animal matter from stock yards and 'of mineral 

 matter which is taken from certain soil deposits containing 

 the desired elements. 



37. Humus. The partially decayed vegetable matter in 

 the soil is usually called humus. The term humus, however, 

 as commonly used, has so many different meanings that it is 

 confusing. On this account the term vegetable matter is 

 used because it includes the fresh supplies of vegetable 

 matter such as roots, stems and manure as well as that which 

 has been partially decayed. The properties usually credited 

 to humus are found also in the fresher forms of vegetable 

 matter. In addition to supplying a source of plant food, it 

 has considerable effect on crop growth in other ways. Soils 

 which contain plenty of vegetable matter are easier to work 

 than those which are lacking in it, for they hold moisture 

 better and are less likely to bake and become cloddy. The 

 acid developed by the decomposition of vegetable matter 

 helps to dissolve some of the mineral matter in the soil and 

 make it available for plants. The dark color of soils is due 

 largely to the presence of an abundance of humus. As dark 



